Simple 2-wire bus buffer
clocking when communicating with them. The buffer includes hysteresis to ensure clean
switching signals are output, especially with slow rise times on high capacitively loaded
buses. Output ports are open-drain type and require external pull-up resistors.
7.3 SDA_IN, SDA_OUT — data signal inputs/outputs
The data signal buffer is bidirectional. The port (SDA_IN, SDA_OUT) which first falls
below the ‘lock voltage’ Vlock, will take control of the buffer direction and ‘lock out’ signals
coming from the opposite side. As the ‘input’ signal continues to fall, it will then drive the
‘output’ side LOW. Again, hysteresis is applied to the buffer to minimize the effects of
At some points during the communication, the data direction will reverse, e.g., when the
slave transmits an acknowledge (ACK), or responds with its register contents. During
these times, the controlling ‘input’ side will have to rise back above the unlock voltage
(Vunlock) before it releases the ‘lock’, which then allows the ‘output’ side to gain control,
and pull (what was) the ‘input’ side LOW again. This will cause a ‘pulse’ on the ‘input’ side,
which can be quite a long duration in high capacitance buses. However, this pulse will not
interfere with the actual data transmission, as it should not occur during times of clock line
transition (during normal I2C-bus and SMBus protocols), and thus data signal set-up time
requirements are still met. Ports are open-drain type and require external pull-up resistors.
7.4 Enable (EN) — activate buffer operations
The active HIGH enable input (EN) can be used to disable the buffer, for the purpose of
isolating sections of the bus. The IC should only be disabled when the bus is idle. This
prevents truncation of commands which may confuse other devices on the bus. Enable
(EN) may also be used to progressively activate sections of the bus during system
start-up. Bus sections slow to respond on power-up can be kept isolated from the main
system to avoid interference and collisions. The pin must be externally driven to a valid
7.5 Direction (DIR) — clock buffer direction control
The direction input (DIR) is used to change the signal direction of the SCL ports. When the
DIR pin is logic LOW, the clock signal input is SCL_IN and the buffered output is
SCL_OUT. When the DIR pin is logic HIGH, the clock signal input is SCL_OUT and the
buffered output is SCL_IN. The pin must be externally driven to a valid state.
Product data sheet
All information provided in this document is subject to legal disclaimers.
Rev. 1 — 25 February 2011
© NXP B.V. 2011. All rights reserved.
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